A bunk bed can efficiently use more space than a single bed. A bunk bed is usually detachable, comprising bed boards, a ladder and two side bedsteads. The side bedstead of the bunk bed is usually an integrate piece. The top and bottom bed share least a support bar or pipe, leading to a large occupied space, and transportation inconvenience. There is an existing bunk bed with the support bar or pipe of the side bedstead being divided into two sections, the support bar or pipe is detachably connected at the centre, so that the bunk bed can be divided into two single beds to be used independently. The side bedsteads can be connected to form a bunk bed. When two side bedsteads is divided into two parts, the connecting structures are increased, the strength of the support bar or pipe of the bunk bed is affected. In general, the consumers do not use the bunk bed separately as two single beds. Therefore, this kind of bunk bed increases the assemble effort of the support bars or pipes, and the divided structure is not a good design.